History of Flight
The aircraft departed Belfast for Southampton and climbed to FL190. With the autopilot engaged and the First Officer handling, a 'TRIM L' amber caption illuminated. The aileron trim indicator showed neutral. Conditions were VMC with no airframe icing and an Outside Air Temperature of -20°C. The aircraft had passed through a moist layer between 1,000 and 6,500 feet. The First Officer disengaged the autopilot and found the aileron trim control immovable; substantial control wheel force was needed to prevent a roll to the right.
The crew consulted checklists but found none applicable. The autopilot trim power switch was turned off with no effect. They decided to divert to Belfast, issued a PAN call, and received radar vectors. During descent, roll controllability improved between FL160 and FL140. The captain took over handling at FL140, and the controls became normal about 10 minutes after the warning. The landing at Belfast was uneventful.
The flight data recorder showed the aircraft at FL190 at 218 kt with autopilot engaged. Before autopilot disengagement, two roll excursions to the right occurred, reaching 10° and 7° over 80 seconds.
Aircraft Description
Primary roll control is via ailerons operated mechanically. A trim tab on the left aileron is adjusted by an irreversible screwjack actuator. The flight deck trim wheel operates a cable system driving a screwshaft. The grease specification had been changed from Aeroshell Grease 7 to Mobil 28 in December 1997. The Component Maintenance Manual listed over-greasing as a probable cause of sticking.
The autopilot provides roll inputs via a servo. If an out-of-trim condition persists for 15 seconds, a 'TRIM L' or 'TRIM R' caution illuminates.
Other Similar Occurrences
Four other occurrences on G-MAJE and one on G-MAJG (another Jetstream 4100) were reported:
- 26 May 1998: Slight lateral trim imbalance; aileron trim control immovable; control freed during descent.
- 17 June 1998 (first): 'TRIM L' caution; jammed trim control; unjammed as Total Air Temperature rose to 0°C.
- 17 June 1998 (second): Stiff aileron trim in cruise after descent through heavy rain.
- 31 October 1998: Immovable aileron trim at FL190; control became easier during descent.
- 30 July 1997 (G-MAJG): Sudden aileron stiffness and roll asymmetry on approach; TAT 4°C; icing suspected; no further issues after lubrication.
Component Investigation
After the 26 May 1998 incident, grease in the aileron trim actuator was found contaminated with moisture; the translating shaft was cleaned and regreased. Following the 4 June 1998 incident, the screwjack was lubricated and functioned normally. Additional lubrication occurred after the 17 June incident. A C Check on 10 July 1998 included strip inspection and relubrication with no faults found. The cable/pulley system was inspected on 26 June 1998 with no faults.
The operator issued Technical Instruction No 72 on 16 July 1998, recommending lubrication of the screwjack in both fully extended and retracted states, with excess grease cleaned after each lubrication.
Discussion
The evidence suggested that jamming of the aileron trim control was probably caused by freezing of water within the trim actuator. Over-greasing was considered unlikely to cause intermittent effects. Water could enter the actuator via clearances as the unit was not sealed. The aircraft manufacturer considered it probable that water had entered when the originally specified grease was in use and had not been purged during subsequent maintenance.
No definitive cause for the lateral trim imbalance could be established; it possibly resulted from the effects of a change in airspeed.